Three peptide fragments of apolipoprotein A-II corresponding to residues 17-31, 12-31, and 7-31 have been synthesized by solid-phase techniques and purified to apparent homogeneity. Each of these fragments contains residues 18-30, a region previously proposed to possess potential amphipathic helical properties. Secondary structural changes of these synthetic fragments accompanying their interaction with phospholipid have been studied by circular dichroism. The magnitude of this interaction has been evaluated from the yields and stoichiometry of lipid-protein complexes isolated by density gradient ultracentrifugation. Fragment 17-31, the smallest peptide containing the proposed amphipathic helix, did not interact with dimyristoylphosphatidylcholine (DMPC) single bilayer vesicles at 24 °C; upon addition of DMPC, no ellipticity change could be detected nor could a stable lipid-peptide complex be isolated. However, fragments 12-31 and 7-31 did interact with phospholipid; in the absence of lipid, both fragments had primarily disordered structures, but when isolated as DMPC-peptide complexes, both fragments possessed increased helical structure. The phospholipidtpeptide molar ratio was 14:1 for fragment 12-31 and 27:1 for fragment 7-31. Studies of space-filling models of these fragments suggest that hydrophobicity and/or length are important properties of phospholipid binding apoproteins.
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